1. Technical Field
The present invention relates to electrochemical corrosion potential sensor, and particularly to an electrochemical corrosion potential sensor suitable for measuring an electrochemical corrosion potential of metallic member wetted by cooling water in a nuclear reactor during operation of a nuclear power plant.
2. Background Art
From the standpoint of improving operation rate of a nuclear power plant, critical problems include suppression of stress corrosion cracking and flow accelerated corrosion of structural materials (stainless steel and nickel base alloy) constituting reactor internals and pressure boundary components and materials (stainless steel, low-alloy steel, and carbon steel) constituting piping.
The stress corrosion cracking occurs when three factors, that is, material, stress, and corrosive environment meet specific conditions, and it can be mitigated by improving one of the three factors in the specific conditions. The flow accelerated corrosion also occurs when material and corrosive environment meet specific conditions, and it can be mitigated by improving one of the factors in the specific conditions.
Hydrogen water chemistry is one of the conventional technologies for improving the corrosive environment for the stress corrosion cracking. In a nuclear reactor, oxygen and hydrogen peroxide are generated due to radiolysis of the cooling water by neutrons and gamma rays. The cooling water containing oxygen and hydrogen peroxide forms a corrosive environment. The hydrogen water chemistry is the technology where feed water into which hydrogen has been injected is supplied into the nuclear reactor so that the hydrogen is reacted with the oxygen and hydrogen peroxide contained in the cooling water to reduce the concentration of the oxygen and hydrogen peroxide in the cooling water. The hydrogen water chemistry is the technology for improving the corrosive environment in the nuclear reactor. On the other hand, an oxygen injection is a technology for improving the corrosive environment concerning the flow accelerated corrosion. When the oxygen concentration in feed water decreases to 10 μg/L or less, the feed water pipe made of carbon steel is thinned down due to the flow accelerated corrosion by the feed water. The oxygen injection is the technology injecting oxygen into the feed water to suppress the flow accelerated corrosion of the feed water pipe.
The improvement effect of the corrosive environment improvement technologies is confirmed, for example, by measuring the electrochemical corrosion potential of targeted structural materials. For a boiling water reactor, the occurrence of the stress corrosion cracking is reported to be suppressed when the electrochemical corrosion potential is reduced to −0.23 V (SHE) or less (refer to R. L. Cowan, et al., “Experience with hydrogen water chemistry in boiling water reactors”, Water Chemistry of Nuclear Reactor Systems 4, Bournemouth, U.K., Oct. 13-17, 1986, Vol. 1, p. 29 (1986)). Moreover, the occurrence of the flow accelerated corrosion is reported to be suppressed when the electrochemical corrosion potential is increased to −0.2 V (SHE) or more by the oxygen injection (refer to Tomonori Satoh, et al., Atomic Energy Society of Japan, Autumn Conference 2005 at Hachinohe Institute of Technology, Sep. 13-15, 2005, p. 458 (2005)). The potential expressed by V (SHE) means the potential relative to the standard hydrogen electrode.
The improvement effect of the corrosive environment improvement technologies is preferably confirmed by measuring the electrochemical corrosion potential throughout the operation cycles, because the flow condition of a plant may change during the operation of the nuclear power plant to vary the electrochemical corrosion potential. For that reason, the electrochemical corrosion potential is preferably measured for at least one operation cycle.
The electrochemical corrosion potential sensor (hereafter, referred to as ECP sensor) needs to generate a constant potential under use environment, and the electrode of the sensor needs to be electrically isolated from a measurement position. The corrosive potential is measured by an electro-meter as a potential difference between the measurement position and the ECP sensor. The ECP sensor used for measuring the corrosive potential is disclosed in for example, Japanese Patent Laid-open No. Hei 3(1991)-17545. The Japanese Patent Laid-open No. Hei 3(1991)-17545 describes the ECP sensor using platinum as its electrode. The ECP sensor generates the constant potential by generating a redox reaction of hydrogen on the surface of the platinum.
The ECP sensor has a structure in which the electrode and a metallic housing are electrically isolated by an insulator disposed between the electrode and the metallic housing. As the insulator, ceramics tolerable to the high-temperature environment and the radiation environment of the boiling water reactor are used. Connection between the electrode and the insulator and between the insulator and the metallic housing is performed by brazing after a surface of a connection section of the insulator is treated by placing thin metallic films on it (called metallization). Methods for suppressing the corrosion of the brazing parts are disclosed in Japanese Patent Laid-open No. Hei 11(1999)-148909 and Japanese Patent Laid-open No. 2002-116281. The Japanese Patent Laid-open No. Hei 11(1999)-148909 discloses a technology for suppressing the corrosion of the brazing part, in which a ceramics layer is placed on the brazing part to prevent the brazing part from directly coming in contact with high-temperature water. In addition, the Japanese Patent Laid-open No. 2002-116281 discloses a technology for suppressing the corrosion of the brazing part, in which platinum as a corrosion-resistant material is plated on the brazing part.